1. Field of the Invention
The present invention relates to a method and apparatus for controlling an amount of fuel to be injected into a diesel engine, and more particularly to a method and apparatus that can reduce or prevent generation of smoke upon steep increase of engine revolution speed.
2. Description of the Related Art
Conventionally, a basic amount of fuel injection in a diesel engine is determined based on an engine revolution speed and an accelerator opening degree, and this basic value is modified based on temperature of water flowing in the engine and intake air temperature in order to determine an ultimate (or target) amount of fuel injection.
With such modification, however, a larger amount of fuel tends to be injected relative to an amount of intake air when the engine runs under a heavy load condition or a vehicle cruises at high altitude. This often results in generation of smoke from the engine. In order to prevent it, Japanese Patent Application Laid-Open Publication Nos. 54-111015 and 7-151007 published Aug. 31, 1979 and Jun. 13, 1995 respectively proposed a technique of limiting an amount of fuel injection within a certain range. Specifically, an intake air sensor detects an amount (flow rate) of intake air, and a maximum amount of fuel injection (limit value) that does not allow smoke to be emitted more than a predetermined value relative to the amount of intake air is determined. Subsequently, the limit value is compared with a basic amount of fuel injection, and the smaller one is selected as the target amount of fuel injection. By doing so, the target amount of fuel injection is controlled to be always smaller than the predetermined upper limit. Accordingly, an amount of smoke generated from the engine stays within a prescribed range under any engine operating condition.
Incidentally, a vehicle driver sometimes stamps an accelerator pedal intentionally in order to promote warming up of the engine soon after the engine is started up. In this situation, the vehicle is still on the road (vehicle speed is zero) and a transmission gear position is neutral. Since the gear position is neutral, the engine is running under no load so that the engine revolution speed steeply increases upon stamping of the accelerator pedal.
At this point, the following problems will occur. Referring to FIG. 6 of the accompanying drawings, when the accelerator pedal opening Ac increases quickly, an amount of fuel injection Q (solid line) correspondingly increases quickly, and the engine revolution speed Ne (solid line) increases steeply with a certain delay. This temporarily reduces an amount of intake air admitted into a combustion chamber of the engine (referred to as "actual MAF" (Mass Air Flow rate)) and a relative amount of fuel Q to the actual MAF becomes larger, thereby generating smoke.
Although the maximum amount of fuel injection is determined from a detection value of the intake air sensor (i.e., MAF sensor), this sensor is located near an inlet of an intake air pipe apart from the engine combustion chamber so that the detection value (MAF sensor value) has a delay relative to the actual MAF (.DELTA.t in FIG. 6). Because of this, if the maximum amount of fuel injection is determined based on the MAF sensor value and the target amount of fuel injection is controlled with such maximum value, the controlling is performed with a certain delay and tolerates emission of smoke.